1. Field of the Invention
The invention relates to the field of networks and particularly to a wireless local area network (WLAN) transmitter.
2. Description of the Related Art
Many devices currently have the capability to communicate without the use of a wired network. Such devices can include, for example, laptops and personal digital assistants (PDAs). These devices can use a wireless local area network (LAN), which can operate separately from or operate in conjunction with an existing wired network.
Although wireless communication typically involves only one transmitter, various communication modes can involve different numbers of receivers. For example, a unicast transmission is a transmission from one transmitter to one specified receiver. In contrast, a broadcast transmission is a transmission from one transmitter to all receivers in a given area, whereas a multicast transmission is a transmission from one transmitter to a specified group of multiple receivers. Note that a transmitter (or a receiver) could be an access point or a client in accordance with standard characterizations. The term “station”, as used herein, can generically refer to either a transmitter or a receiver.
In wireless communication, messages can be transmitted as packets of data over a channel, wherein a packet has a header (e.g. including the transmitter's and receiver's addresses) as well as data. Note that the channel can be defined by one or more characteristics (e.g. a frequency and/or a modulation scheme). In packet switching, the transmitter transmits each packet individually over the channel to its destination.
For example, FIG. 1 illustrates a transmitter 110 transmitting multiple streams of communication 111, 112, and 113 over a channel 130 to a receiver 120. In one embodiment, these multiple streams of communication 111, 112, and 113 could provide different applications, e.g. video, data, and audio, thereby requiring separate queuing for each stream to ensure quality of service (QoS). In FIG. 1, each of the streams of communication 111, 112 and 113 transmitted over channel 130 may have multiple packets being transmitted, with the packets illustrated closest to the receiver 120 being received first in time. For example, a communication 100 could comprise a plurality of serially-transmitted packets 101-107, wherein communication stream 111 includes video packets 101-103 in sequence, communication stream 112 includes data packets 104-106 in sequence, and communication stream 113 includes an audio packet 107.
Many wireless communication schemes, such as those based on the IEEE 802.11 standards (including the IEEE 802.11a, 802.11b, and 802.11g standards), can be implemented using a range of data rates. For example, the IEEE 802.11b standard enables data rates from 1 Mbps to 11 Mbps, whereas the IEEE 802.11a standard enables data rates from 6 Mbps to 54 Mbps. Due to various features of the encoding schemes dictated by many such wireless communication standards, transmission on a channel is generally established at a selected available data rate within an available set of rates.
For example, depending on channel conditions, reliable communication in compliance with the IEEE 802.11a standard may take place at 6 Mbps, 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps, and 54 Mbps. In a similar manner, the IEEE 802.11b standard includes data rates of 1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps. The IEEE 802.11g standard can support rates from both 802.11a and 802.11b. Note that other intermediate, slower, or faster data rates may be possible either currently or in the future. Thus, the embodiments herein using specific data rates are illustrative only and not limiting.
Lower data rates typically allow for more reliable transmissions in challenging environments, e.g. noisy, distant, or otherwise flawed channels. Higher data rates can be used across ideal or nearly ideal channels. As used herein, the term data rate refers to the amount of information that can be successfully transmitted by transmitter 110 in a given time period. The data rate may be disadvantageously reduced due to signal degradation in channel 130. As more signal degradation occurs, the receiving device of receiver 120 may only be able to receive accurately data transmitted more slowly, thus requiring more time to transfer the same amount of data. Because the transmitting device and the receiving device are set to the same data rate, the lower the data rate of the transmitting device the more time the receiving device has to read the data. However, for maximum throughput, it is desirable to determine the highest, reliable data rate available.
Unfortunately, dynamic conditions present in the channel can degrade signal quality. For example, one reason for signal degradation is shadow fading, wherein objects near either the transmitter or the receiver (e.g. walls, people, cars) can block the signal. Another reason for signal degradation is distance-dependent path loss, wherein the signal experiences a reduction of power with increasing distance. Another reason for signal degradation is multi-path propagation, wherein echoes of a single signal (e.g. caused by reflection) can result in the superposition of that signal. Yet another reason for signal degradation is interference, wherein other signals in the vicinity of a first signal can interfere with that first signal, thereby degrading its quality. Note that these other signals could be from either 802.11 compliant devices that are too far away to know to be quiet, but close enough to affect a given signal when they transmit, or from non-compliant devices (e.g. devices using different standards).
Current schemes fail to effectively account for these dynamic changes in the channel. For example, in one scheme, a system administrator can determine (or at least estimate) the distance between stations and set the data rate according to this distance. In other words, if the stations are close together, then the data rate can be set relatively high as the likelihood of signal degradation is low. In contrast, if the stations are far apart, then the data rate can be set relatively low as the likelihood of signal degradation is high. Unfortunately, this scheme fails to take into account actual channel conditions, which can vary significantly irrespective of distance between stations.
In another scheme, if the packet error rate (i.e. the number of packets that fail to reach the receiver divided by the total number of packets sent by the transmitter) exceeds a predetermined threshold, then the data rate for subsequent packets could be adjusted to the next lower data rate. In contrast, if the packet error rate falls below another predetermined threshold, then the data rate for subsequent packets could be adjusted to the next higher data rate. These predetermined thresholds provide a sub-optimal solution that worsens with the number of available data rates. In other words, for a limited number of data rates, the fixed thresholds based on packet error rate provide a rough, but relatively reasonable indication of when to change to another data rate. However, in current systems where numerous data rates are available, the fixed thresholds based on packet error rate can become too rough for meaningful adjustment of the data rate. Also, this scheme is too slow for adapting in an indoor, multipath environment that has quickly changing channel conditions.
Therefore, a need arises for a system and method of automatically optimizing utilization of the wireless channel.